Structural health monitoring (SHM) of composite structures made from fiber reinforced polymer (FRP), concrete, and/or other materials can improve the safety of physical infrastructure. FRP composites, in particular, continue to find increasing use in structural applications in the aerospace and marine industries, as well as in wind turbine blades and civil infrastructural systems. This has largely been enabled by their relatively high strength to weight ratios, and excellent fatigue and corrosion resistance. They are however susceptible to low velocity impact resulting in barely visible impact damage (BVID). Similarly, key civil infrastructure systems (CIS) like bridges, dams, roads and buildings made of cementitious composites like concrete are continually exposed to natural degradation because of old age, over loading and seismic activities. They are also exposed to man-made damage from terrorist attacks and impact collisions.
There are currently a number of techniques being used for damage detection and monitoring of composite structures. These include imaging techniques such as ultrasonic cross sectional scan (C-scan), x-ray, and thermography. These techniques, however, do not provide for in-situ sensing of damage and/or fractures, and as a result, these techniques do not allow for reliable real-time monitoring of structures. Furthermore, the associated costs resulting from the downtime required for periodic non-destructive inspections can be very high for aerospace structures like aircrafts and civil structures like bridges. Acoustic emission techniques are promising, but suffer from low-signal-to-noise ratio. Surface-mounted resistive foil strain gages have potential for in-situ and continuous monitoring, however, they are less effective in monitoring internal delamination and/or damage and are vulnerable to electromagnetic and electrical interference, as well as physical damage.
Triboluminescence (TL) is the emission of light from materials when stressed, pulled apart, ripped, scratched, rubbed, or fractured. There have been a number of attempts to apply the TL phenomenon for damage sensing in composite structures. A challenge of applying TL-based sensor systems is the ability to effectively capture and transmit an optical signal generated within opaque composites like concrete and carbon fiber reinforced polymers. Other factors critical to the effective implementation of TL damage sensing include the effective dispersion of TL materials and the determination of optimized concentration levels in the host materials.
An approach being employed is to incorporate TL crystal into resin and infuse through the fiber layup in which optical fibers (for signal transmission) have been placed. The whole part is then cured. There are however many limitations with this approach. Firstly, the concentration level of the TL crystals in the host matrix required for good TL response is usually high. This introduces parasitic weight effect which is highly undesirable. The density differences between the TL crystals and resin matrix may also cause the settling of the crystals during curing. There will therefore be uneven dispersion of the crystals in the cured composite part or structure. Another problem associated with this approach is filtration during the infusion process.
An alternative approach requires the incorporation of TL crystals in a polymer to produce thin films. The films are then placed on the surface of composite laminates to monitor surface damage. In this case however, signal transmission may be an issue. Furthermore, placing the thin film within a laminate may promote delamination at the laminate interfaces.